Ohio’s ancient sea monster, the Dunkleosteus terrelli, is revealed in new clarity by a recent study that shows just how strange the creature truly was.
Researchers from Case Western University led the work, which appeared in The Anatomical Record. The 14-foot-long armored sea monster has intrigued paleontologists since its initial discovery in 1860, as it possessed distinctive razor-sharp bone blades rather than teeth, revealing a shark-like fish that was an apex predator during the Late Devonian period.
The odd-looking sea monster, with its hard, bone-plated skull, is a notable fixture in museums worldwide. While its appearance will be familiar to those who have taken an interest in such ancient creatures, its fame is minimal, and researchers have long neglected the extinct predator. Among the Case Western Reserve team’s findings was that Dunkleosteus was highly unusual among arthrodires, the group to which it belongs.
Dunkleosteus Terrelli
“The last major work examining the jaw anatomy of Dunkleosteus in detail was published in 1932, when arthrodire anatomy was still poorly understood,” said lead author Russell Engelman, a graduate student in biology at Case Western Reserve. “Most of the work at that time focused on just figuring out how the bones fit back together.”
A significant challenge to understanding the arthrodire group is their exoskeleton-based anatomy. Like modern sharks, arthrodires’ inner skeletons were mostly cartilage, which did not survive the ages. Instead, what remains is their bony head and torso armor, often found crushed and flattened, leaving many questions about their original anatomy.
“Since the 1930s, there have been significant advances in our understanding of arthrodire anatomy, particularly from well-preserved fossils from Australia,” Engelman said. “More recent studies have tried biomechanical modeling of Dunkleosteus, but no one has really gone back and looked at what the bones themselves say about muscle attachments and function.”
Sea Monster Investigations
The international effort drew scientists from Australia, Russia, the UK, and the US to study specimens housed at the Cleveland Museum of Natural History, which houses the largest and most well-preserved Dunkleosteus fossil collection in the world. Such a collection makes sense for the museum, as Cleveland’s unique Devonian-era environmental conditions enabled optimal preservation. For this reason, the area hosts a large portion of the ancient global predator’s preserved remains in black shale rock that was once the sea floor. Now, natural erosion and infrastructure projects are exposing that ancient sea floor and with it, the ancient fossil remains of Dunkleosteus.
The team made several notable discoveries in their detailed analysis of the Cleveland Museum collection. Dunkleosteus’ skull is comprised of much more cartilage than previously understood, including most major jaw connections and muscle attachment points. The creatures were even more similar to modern sharks than previously believed, housing a similar facial jaw muscle in a large bony structure.
Arthrodires Reconsidered
Dunkleosteus also diverged considerably from other arthrodires, losing the group’s typical teeth in favor of its bone blades over the course of evolution. Arthrodire evolution broadly favored specialization in hunting large fish, but only Dunkleosteus and its closest relatives produced the distinctive bone blades for tearing large chunks of flesh from their prey.
“These discoveries highlight that arthrodires cannot be thought of as primitive, homogenous animals, but instead a highly diverse group of fishes that flourished and occupied many different ecological roles during their history,” Engelman said.
The work reinforces the notion that some of the great paleontological discoveries remain to be made, not in fieldwork, but in careful reexamination and comparison of fossils long stored in forgotten drawers in institutional collections.
Case in point, even within one well-known museum’s most famous collections, an unknown chapter in the story of the evolution and diversity of a remarkable ancient species was waiting to be told.
The paper, “Functional Anatomy, Jaw Mechanisms, and Feeding Behavior of Dunkleosteus terrelli,” appeared in The Anatomical Record on November 20, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.